Streptococcus gordonii, a prominent commensal bacterium of the human oral cavity, can be associated with odontogenic infections as well as infective endocarditis. Enterococcus faecalis, predominantly a component of gut microflora, has emerged as an opportunistic pathogen, known to carry multiple plasmids encoding clinically significant virulence factors and antibiotic resistance. E. faecalis is also found in the oral cavity and is of particular significance in endodontic infections where it is commonly recovered from pulp canals with failed endodontic treatment and persistent infection. Transfer of mobile DNA elements between S. gordonii and E. faecalis has been demonstrated in vitro and in an ex vivo tooth model. Since both organisms inhabit the oral cavity and have been recovered from recalcitrant endodontic infections, such DNA transfer has the potential to occur in vivo. E. faecalis secretes multiple hepta- and octameric peptide pheromones that induce a mating response in enterococci harboring specific families of conjugative plasmids. The peptides induce a clumping/mating response in plasmid-carrying responder cells, involving expression of cell-surface proteins involved in inter-cellular aggregation and DNA transfer functions, thereby facilitating inter-cellular contact and plasmid transfer. This process is well-studied between plasmid-bearing enterococcal donors and plasmid-free enterococcal recipients. Plasmid pAM373 is an enterococcal conjugative plasmid that responds to its cognate pheromone cAM373 secreted from plasmid-free enterococci. Derivatives of pAM373 that carry resistance to multiple antibiotics including a vancomycin, are also cAM373-responsive. Plasmid pAM373 is novel in that it also responds to pheromone-like signals from S. gordonii. We have recently identified the gene, camG, that encodes the S. gordonii heptapeptide gordonii-cAM373. Synthetic gordonii-cAM373 was able to induce a clumping response in E. faecalis and induce plasmid transfer from E. faecalis to S. gordonii. This is the first demonstration that a peptide from a streptococcal species can act as an E. faecalis mating signal and facilitate intergeneric plasmid transfer. These results have intriguing implications for a potential role of streptococcal pheromone-like peptides in intergeneric DNA transfer, and suggest that such peptides could provide signals by which E. faecalis could contribute to the reservoir of virulence and resistance determinants in the commensal oral flora. Conversely, this demonstration of a streptococcal signal that induces expression of E. faecalis putative virulence factors suggests that the presence of oral streptococci in polymicrobial infections may increase the pathogenic potential of E. faecalis. We propose to further investigate this novel intergeneric cell-to-cell communication via the following Specific Aims: 1) to further characterize the molecular mechanisms involved in S. gordonii camG expression and processing and determine environmental conditions that favor or disfavor its expression; 2) To monitor the response of E. faecalis virulence genes when exposed to S. gordonii pheromone-producing cells and gordonii-cAM373 peptide.